(a) Field of the Invention
The invention relates to a lamp driving system, and more particularly, to an inverter circuit for driving a discharge lamp of a liquid crystal display panel with a feedback loop for adjusting a current flowing through the lamp.
(b) Description of the Prior Art
A discharge lamp, especially a cold cathode fluorescent lamp (CCFL), has excellences of high efficiency and law cost, and is therefore extensively applied to liquid crystal displays (LCD) to serve as a light source of a backlight system. An inverter circuit is used for driving such CCFL, and is capable of supplying an extremely high excitation voltage as well as reducing the supply voltage to a smaller operating voltage when the lamp is illuminated.
Referring to FIG. 1 showing a schematic circuit diagram of a conventional lamp driving system, an inverter 10 comprises a driving circuit 12 and a transformer 14. The driving circuit 12 is for converting a DC power source to an AC signal that are boosted by the transformer 14 to produce an AC power source further forwarded to a lamp 20. At this point, the inverter 10 has an output voltage VOUT and an output current IOUT.
To accurately control brightness of the lamp 20, and taken into consideration that brightness of the lamp is approximately proportional to a current flowing through the lamp, a lamp driving system is provided with a current feedback loop as basis for adjusting the current of the lamp. Generally, the feedback loop uses a pulse-width modulation (PWM) controller 16 to produce a feedback control signal to the driving circuit 12 based on IOUT sampled from a secondary side of the transformer 14. The feedback control signal thus controls duty cycles of the driving circuit 12 so as to adjust an average output current of the inverter 10.
However, as shown in FIG. 1, inherent parasitic capacitance C1 is present at the lamp 20. In addition, when the lamp 20 is installed to a housing of the LCD panel, between any high voltage terminals (lamp) to a ground terminal (panel housing) is distributed stray capacitance C21, C22 . . . C2n—the parasitic capacitance respectively leads to leakage currents I1 and I2. It is concluded that, in the lamp driving system shown in FIG. 1, the current IOUT outputted from the inverter 10 is not actually the current IL flowing through the lamp; instead, the current IOUT is a sum of the lamp current IL, and the leakage currents I1 and I2.
The parasitic capacitance increases as the length of the lamp lengthens, and the larger the parasitic capacitance is, the higher the leakage current gets. Wherein, the leakage current I2 especially has a greater influence. Moreover, when the lamp 20 is installed to the housing of the LCD panel, even minute errors of installation lead to a significant inherent stray capacitance differences. Under normal circumstances, the leakage current I2 may be as high as 30% to 50% of the output current IOUT of the inverter 10.
FIG. 2 shows a waveform diagram of relative voltage and current signals of the lamp driving system circuit in FIG. 1. The lamp is a resistive load, with the current IL and the voltage VOUT of the high voltage terminal of the lamp being same phase, and the leakage currents I1 and I2 having 90 degrees phase difference from the voltage VOUT. Therefore, a phase difference between the current IOUT and the voltage VOUT ranges between 0 to 90 degrees.
In the conventional lamp driving system shown in FIG. 1, a feedback sampling method samples within an extremely short time at the peak point P1 of the current IOUT as shown in FIG. 2, and another feedback sampling method samples during an entire positive semi-circle of the current IOUT as shown in FIG. 2(e). Regardless which method is adopted, the sampled current contains a certain percentage of leakage current, and the actual current IL flowing through the lamp remains unobtainable. Thus, such feedback control method fails to ensure precise brightness control to incur noticeable brightness differences of the lamp.
To overcome the aforesaid drawbacks, another conventional lamp driving system in FIG. 3 is used. Referring to FIG. 3, a low voltage terminal of a lamp 20 serves as a feedback point to form a feedback loop. In such conditions, a sampled current received by the PWM controller 16 is IL+I 1. Although influences of the leakage current I2 is eliminated, this method is yet is incapable of sampling the actual current IL of the lamp in a most precise manner. Furthermore, in numerous designs of LCD panels, such sampling feedback method that samples from low voltage terminal of the lamp is not permitted. Therefore, it is vital to develop other feedback control techniques for solving the aforesaid issues.